I want to call SaveChangesAsync and when this succeeds, clear a list.

You don't need ContinueWith, you can just write it like this

public async Task<int> CommitAsync()
{
    var result = await DataContext.SaveChangesAsync();
    _addedEntities.Clear();
    _changedEntities.Clear();
    _deletedEntities.Clear();
    return result;
}

It's poor practice to throw an Exception as it means client code cannot be specific about the exceptions it catches. I would suggest changing RethrowException to a method that constructs the exception message you want, then calling it like this:

public async Task<int> CommitAsync()
{
    try
    {
        var result = await DataContext.SaveChangesAsync();
        _addedEntities.Clear();
        _changedEntities.Clear();
        _deletedEntities.Clear();
        return result;
    }
    catch (DbEntityValidationException e)
    {
        var exceptionMessage = GetDbEntityValidationExceptionMessage(e);
        throw new DbEntityValidationException(exceptionMessage, e);
    }
}

This also removes the return default(int); which is just there to appease the compiler.

I want to call SaveChangesAsync and when this succeeds, clear a list.

You don't need ContinueWith, you can just write it like this

public async Task<int> CommitAsync()
{
    var result = await DataContext.SaveChangesAsync();
    _addedEntities.Clear();
    _changedEntities.Clear();
    _deletedEntities.Clear();
    return result;
}

In the original code, the catch block will never run.

Consider what happens when SaveChangesAsync throws a DbEntityValidationException: antecedent.Result will throw an AggregateException wrapping the DbEntityValidationException.

It's poor practice to throw an Exception as it means client code cannot be specific about the exceptions it catches. I would suggest changing RethrowException to a method that constructs the exception message you want, then calling it like this:

public async Task<int> CommitAsync()
{
    try
    {
        var result = await DataContext.SaveChangesAsync();
        _addedEntities.Clear();
        _changedEntities.Clear();
        _deletedEntities.Clear();
        return result;
    }
    catch (DbEntityValidationException e)
    {
        var exceptionMessage = GetDbEntityValidationExceptionMessage(e);
        throw new DbEntityValidationException(exceptionMessage, e);
    }
}

This also removes the return default(int); which is just there to appease the compiler.

RethrowException is a worrying name for a method, but I'll get to that later.

I want to call SaveChangesAsync and when this succeeds, clear a list.

You don't need ContinueWith, you can just write it like this

public async Task<int> CommitAsync()
{
    var result = await DataContext.SaveChangesAsync();
    _addedEntities.Clear();
    _changedEntities.Clear();
    _deletedEntities.Clear();
    return result;
}

Now, what is the point of the try-catch in the original code? Judging by the name, RethrowException will ... throw an exception. So it looks like the return default(int); line is just there to appease the compiler. If you want to log an exception before throwing, write it like this:

public async Task<int> CommitAsync()
{
    try
    {
        var result = await DataContext.SaveChangesAsync();
        _addedEntities.Clear();
        _changedEntities.Clear();
        _deletedEntities.Clear();
        return result;
    }
    catch (DbEntityValidationException e)
    {
        Log(e);
        throw;
    }
}

See this blog post for a discussion of throw vs throw e.

I want to call SaveChangesAsync and when this succeeds, clear a list.

You don't need ContinueWith, you can just write it like this

public async Task<int> CommitAsync()
{
    var result = await DataContext.SaveChangesAsync();
    _addedEntities.Clear();
    _changedEntities.Clear();
    _deletedEntities.Clear();
    return result;
}

It's poor practice to throw an Exception as it means client code cannot be specific about the exceptions it catches. I would suggest changing RethrowException to a method that constructs the exception message you want, then calling it like this:

public async Task<int> CommitAsync()
{
    try
    {
        var result = await DataContext.SaveChangesAsync();
        _addedEntities.Clear();
        _changedEntities.Clear();
        _deletedEntities.Clear();
        return result;
    }
    catch (DbEntityValidationException e)
    {
        var exceptionMessage = GetDbEntityValidationExceptionMessage(e);
        throw new DbEntityValidationException(exceptionMessage, e);
    }
}

This also removes the return default(int); which is just there to appease the compiler.